The RF-PECVD (radio frequency-plasma enhanced chemical vapor deposition) technology is widely used in the mass industrial production of large-area silicon film photovoltaic solar cells. In order to raise the conversion efficiency of solar cells, nano-crystalline silicon films where nano-crystalline grains are distributed in amorphous silicon are adopted in most cases. However, RF-PECVD technology features low efficiency in deposition of nano-crystalline silicon and low crystallinity. There has been developed the VHF-PECVD (very high frequency—plasma enhanced chemical vapor deposition) method. VHF-PECVD method helps raise the deposition velocity and quality of nano-crystalline silicon films. Exciting plasma by VHF can raise plasma density and reduce the sheath thickness and voltage of substrate surface, thus reducing the ion energy reaching the substrate, increasing the ion flow delivered to the substrate and achieving the double effects of raising deposition velocity and crystallization ratio of the films. Therefore, VHF-PECVD has tremendous potential of technical development in the industry.
In the plasma excited by a VHF electromagnetic field, there is an inherent problem: the increase of the excitation frequency of the electromagnetic field will make the uniformity poorer. Particularly, when one quarter of the vacuum wavelength to which the excitation frequency corresponds is approximate to the scale of the electrodes, the non-uniformity generated by the standing wave effect in the capacity-coupled electrode reaction chamber will be rather serious. Therefore, the potential standing wave effect is considered the most important non-conformity source generated during application of VHF technology in large-area PECVD. In order to obtain uniform plasma and thereby realize uniform deposition, typically the geometric shape of the electrodes is redesigned to improve the distribution of the electric field. Further, multi-point feeding, phase control, superposition of a plurality of power sources, pulse modulation of power sources and other methods may also improve plasma uniformity to some extent.